Ever increasing demands directed to the quality of ball bearings require that the balls should without exception approach the shape of the theoretical sphere. That means, that diameters measured from any position and of any surface smoothness should comply with these requirement. Any single ball with deviating geometry ball in a given bearing, results in untimely wear. Therefore, most stringent control is required, with maintenance of exact characteristics for the entire group of balls in a bearing.
Of all known methods used for supporting, orienting, and controlling of balls that method is considered the most accurate, in which the ball to be measured is placed on three balls of known diameter, of identical group tolerance, and having the same or different diameter of the ball to be measured, serving as reference standard and arranged in a nest. Then the ball to be measured is touched with a diamond needle led thereto, and the distance between a point on the corresponding standard ball and the diamond needle--corresponding to the diameter of the ball to be measured--is measured by inductivity. The balls with radii or diameters beyond the tolerance limit are separated out.
The disadvantage of this method is that only one signal linear dimension (diameter, or radius) is obtained. This is valid between two preferred points of the given sphere. It can happen, however, that measuring between two other "preferred" points would give different measuring results. Therefore, this type of measurement does not sense an eventual deviation from the spherical shape. It can happen, even if only in a small number of cases, that defective balls are evaluated by this method as being accurately shaped balls.
A further disadvantage of this method is that no information can be gained on the surface smoothness of the balls. Furthermore, the diamond tip that mechanically contacts the balls to be measured has to be replaced after a certain number of measurements, involving considerable cost and effort.
Thus, this type of measurement is more suitable for use in the laboratory rather as a technique routinely used in manufacture.